If It Flies ...
Feet-on-the-ground test-pilot school
Hi, gang. It's time to continue our feet-on-the-ground test-pilot school. Test-flying our aircraft is a practice that separates us aeromodelers from all but a relative handful of full-scale pilots: the professional test pilots and the amateurs, otherwise known as home-builders.
We aeromodelers actually share a great deal with the home-builders, including a fair number of people. The overlap between us and full-scale pilots is heavily populated by home-builders, or those who belong to the EAA (Experimental Aircraft Association). That group has experience and information to share with both groups, and many of us would do well to "debrief" these pilots over a friendly cup of coffee. Flying stories are much the same, whether models or full-scale, and descriptively hand-flying serves as a common language.
In some cases, the full-scale home-builders become modelers to gain access to the poor man's wind tunnel, which is also known as the sky. Testing the stability of an airplane near the aft CG limit, spin-recovery testing, and even dive and flutter testing is much less risky if you have done the tests on a scaled-down prototype with your feet safely on the ground.
Full-scale flight-testing is a methodical, step-by-step process, because you need to sneak up on an aircraft's bad behavior without taking reckless, life-endangering risks—even though you may not know exactly where or how the airplane's nastiness will begin. The aircraft's safe-flight envelope is pushed open wider and wider, and when the limits begin to show themselves, the pilot's manual reflects those with some safety margin built in. Flying is generally safe as a result.
Before a full-scale student pilot climbs into a Cessna 152 or similar trainer, the instructor will spend at least a little time discussing the permissible weight-and-balance calculation and chart in the pilot's manual. When pilots fail to adhere to this procedure, we see pictures of the wreckage on TV and read about it in more depth later. For some reason, the spectacular crashes often seem to involve famous passengers who refuse to travel without entourages and heavy baggage.
The professional test pilot's superior piloting skills are called for when the test airplane's behavior deteriorates abruptly and unexpectedly: when slow or incorrect first responses might result in a smoking airplane-shaped hole in the ground. With rare exceptions, methodical testing almost turns the test pilot's job into another day at the office. Of course, the stakes are nowhere nearly as high for us aeromodelers; our feet are safely on the ground. So why approach this test-flight stuff so scientifically?
I don't know about you, but I hate to pound my investment in both time and money into little pieces, especially when I designed the thing. No matter how disposable you might consider your models to be, they do create safety hazards when they are not fully under control.
I'm not looking to step on MA safety columnist Dave Gee's toes, but through the years I have seen many airplanes flown repeatedly in front of crowds despite the fact that they were not properly shaken down in early flight-testing. An instance that is vivid in my mind was a high-wing competition scale airplane that was flown for years in contests near where I live. It veered uncontrollably to the left after each takeoff and overflew the pits as a result. A year later, I watched the same model, and pilot, do the same thing in the same place on the same runway. I changed where I parked the car on year three.
There is no need to poke at this pilot's flying skills, because the airplane obviously needed changes that obviously would have enhanced my safety and his competition flying scores. It has to be tough to overcome getting a zero score on Takeoff on each flight. The changes would have been minor, but it requires adopting the mindset that you will make a small change, hopefully based on the kinds of things we discussed here in the last few months, and then go fly the model with an eye toward critically evaluating what you have accomplished.
If need be, get a flying buddy to watch carefully. Two sets of eyes might be better than one, especially when that first set is busy flying.
The Dreaded Tuck-Under
In the past few columns I've written about engine side thrust and downthrust, roll control, adverse yaw, and the evils of cross-trimming. In each case I covered the flight test. Sometimes it is simple, and other times it takes patience and a few tries. Maybe there is something to those test-pilot skills after all.
A while back I wrote about unbalanced wing weight. If I remember correctly, I covered how an airplane tends to wander off in the direction of the heavy wingtip at the most critical stage of landing. Now let's complete the test-pilot course by reviewing the effects of the fore-and-aft balance point, or center of gravity (CG).
Depending on the type of model and the type of flying you do, different tests are called for when it comes to investigating the effects of CG position. I'll start with the most dramatic one I can think of, only because this one is fun.
Flying wings or tailless aircraft have special stability issues that are shared only by airplanes with tails that have highly cambered wings and short tail moments. When these models are balanced for optimum performance, they become sensitive to high airspeed in a special way.
Normally, assuming that an aircraft is nose-heavy enough for good stability, its pitch trim becomes airspeed-sensitive in the positive sense: the faster it goes, the more it tends to climb. The slower its airspeed, the more it tends to drop its nose. If you think about it, that's a nice, stable state of affairs.
When a flying wing has a forward CG, it will be stable but it will also waste much of its lift fighting the nose-down pitching tendency that the airfoil has. For this reason, flying wings either have trailing-edge reflex or elevons trailed up.
If the flying wing's CG is far enough aft to achieve close-to-optimum performance, the amount of reflex or up-elevon is reduced, as is the drag this creates. The model also develops a critical airspeed above which its normal airspeed/pitch relationship turns backward. With increasing airspeed, the airplane tends to nose down. The resulting dive leads to even higher airspeeds. Left unchecked, the aircraft will restabilize in a high-speed, inverted 45° dive. That usually ends badly.
Sometimes the aircraft responds to up-elevator and a reduction in power (if not a glider), and sometimes it does not. The difference is often in how quickly the correction is made. What to do?
The flight test itself is simple. Do the test at full throttle if you have an engine or motor, because when the tuck-under shows up you can simultaneously pull the power back and pull up-elevator.
- Start the test in full-power, level flight.
- On the next pass, use a shallow (about 5°) dive.
- Make the next dive a little steeper, and then a bit steeper again, until either:
- you are convinced the problem does not occur at those speeds, or
- you are flying fast enough to concern yourself with control-surface flutter.
If the tuck-under appears, pull off power quickly and apply up-elevator to kill the airspeed. Conduct these tests high enough to give yourself some recovery room, and for heaven's sake perform them pointed away from any people.
Those methodical steps I wrote about before are important now. Start noticeably nose-heavy and move the CG aft a tiny bit each time the flight test is successful. Small CG movements assure that the problem won't be severe when it rears its ugly head, and human reflexes will be fast enough to stop the tuck-under before it fully develops. Then go back to the last good CG position.
Maybe 15 years ago, a flying buddy of mine contracted with a local home-builder/designer to produce a stability test model of a flying-wing ultralight he planned to build and for which he hoped to sell plans. Repeated tests showed that the aircraft had a pronounced tuck-under when balanced at the designer's calculated CG.
If the full-scale pilot's arms were not strong enough to counteract the nose-down pitch, this sort of thing could have killed him. As it was, the roughly 2,000-square-inch test model almost overpowered the giant-scale servos, and I nearly lost the airplane the first time we did this test. When we found the real safe aft CG location, the designer got disgusted with the projected performance and abandoned the project. He is still alive, though.
Fine-Tuning Pitch Stability
Most trainers, especially those with flat-bottomed airfoils, are intended to be nice and stable; that is, they tend to nose up when airspeed increases and drop the nose when airspeed decreases from the speed at which the model was trimmed. As I mentioned, that is a good thing.
Too much of a good thing means that the airplane tends to climb too steeply at full power and that the glide ends up too steep for comfort, all with the same elevator trim setting. Curiously enough, this is what a Cessna 152 will do when trimmed for level cruise flight. Full-scale pilots constantly retrim the elevator based on what they are doing.
If the pitch stability is marginal, the full-power climb might require that you add up-elevator, which is no big deal, but the glide angle might be extremely shallow with low airspeed and poor roll-control authority.
The flight tests look like this. Trim the elevator for level flight at the same throttle setting that you normally intend to use for cruising flight. The following checks are done with that same elevator trim setting.
- From level flight, with your hands off the stick (unless a problem develops), add full throttle and observe:
- Is the climb too steep?
- Does the model pitch up, stall, and drop the nose?
- Or does it simply fly faster without climbing much?
- Take note of what the aircraft does and then move on to the power-off test.
Transcribed from original scans by AI. Minor OCR errors may remain.
